The invention relates to a combined installation for the production of biogas and compost from biomass, including at least one fermenter according to the principle of dry fermentation and to a method of switching a fermenter in a like installation between biogas production and composting.
So-called “dry fermentation” allows pourable biomasses from agriculture, from biological waste and from communal cultivated areas to be converted to methane without having to convert the materials to a liquid substrate which can be pumped. Biomasses having a dry substance content of up to 50% can be fermented. This dry fermentation method is described, for example, in EP 0 934 998.
In the case of “dry” fermentation, the material to be fermented is not stirred into a liquid phase as is the case, for example, with liquid fermentation of organic waste. Instead, the fermentation substrate which has been introduced into the fermenter is kept moist all the time by drawing off the percolate at the bottom of the fermenter and spraying it over the biomass again. This results in optimum living conditions for the bacteria. During recirculation of the percolate, the temperature can moreover be regulated, and it is possible to add additives for process optimisation.
From WO 02/06439 a bioreactor or fermenter having the form of a prefabricated garage is known, which is operated according to the principle of dry fermentation in the so-called batch process. In this case, after seeding with already fermented material, the fermenter is filled with the fermentation substrate by means of tractor shovels. The fermentation container is constructed in the form of a garage and is closed by a gastight door. The biomass is fermented with air being excluded, with no further thorough mixing being performed during the process, and with no additional material being supplied. The percolate which seeps out of the material being fermented is drawn off via a drainage groove, is temporarily stored in a tank, and is again sprayed over the fermentation substrate, in order to moisturize it. The fermentation process takes place in the mesophilic temperature range between 34 and 37° C., with temperature equalisation being carried out with the aid of floor heating and wall heating.
The resultant biogas can be used to obtain electricity and heat in a block-type thermal power station (BHKW; Blockheizkraftwerk). In order to ensure that sufficient biogas is always available for the block-type thermal power station, a plurality of fermentation containers are operated with offset timings in the dry fermentation installation. At the end of the dwell time, the fermenter area is emptied completely and then refilled. The fermented substrate is subsequently supplied to composting, resulting in the production of an organic fertiliser that is comparable to conventional composts.
Such fermenters for the production of biogas according to the principle of dry fermentation are further known from DE 203 19 847 U1 and from EP 1 681 274 A2. From DE 34 38 057 it is known to produce compost from the used or fermented biomass from a biogas installation.
Batch operation makes it necessary to shut down the individual fermenters from time to time, i.e., after the biomass present in the fermenter was subjected to complete anaerobic conversion; in other words, the biogas production must be stopped, the fermented biomass must be removed from the respective fermenter, fresh biomass must be charged into the fermenter, and the biogas production has to be resumed. This involves the drawback that it is necessary, for safety reasons, to prevent an explosive biogas/air mixture from being created while the individual fermenters are being loaded and unloaded.
To this end, it is known from EP 1301583 B to flood a fermenter during its operation with waste gas containing carbon dioxide from the block-type thermal power station that is being operated with biogas, in the event of an explosion risk, that is to say if air has entered the fermenter. Subsequently the fermented biomass may be removed without any risk from the fermenter and supplied to a composting installation.
It is therefore the object of the present invention to further develop a biogas installation as known from EP 1301583 B in such as way that post-composting of the spent biomass is simplified.
This object is achieved through the features disclosed herein.
Due to the fact that the spent biomass is composted in the fermenter by switching over from anaerobic fermentation to aerobic composting, it is no longer necessary to convert the spent biomass in a separate composter. A combined installation includes the necessary components in order to enable safe switching, shutting down and unloading, as well as a safe start-up of a fermenter. The fermenter of the invention is configured such that the entire fermentation process, which consists of anaerobic fermentation and aerobic composting, may unfold inside it before it becomes necessary to remove the spent biomass and again charge the fermenter with fresh biomass.
In accordance with a preferred aspect of the invention, a first purging gas inlet opens into the fermenter in the area above the biomass.
In accordance with a preferred aspect of the invention, the fermenter comprises a floor plate having provided therein purging gas passages that are connected to a second purging gas inlet.
In accordance with a preferred aspect of the invention, the purging gas passages are configured for discharging seepage liquids seeping from the biomass during the production of biogas.
Biogas production and processing are maintained for as long as possible even while the fermentation process is terminated by purging with waste gas containing carbon dioxide, i.e., the biogas/waste gas mixture of the fermenter continues to be supplied to the biogas consumer until the quality of this mixture drops below a predetermined degree, before the fermenter is then switched over for composting of the fermented biomass contained in it. Only when the methane concentration in the biogas outlet drops below an upper limit, the biogas line leading to the biogas consumer is disconnected from the biogas outlet. After this, the biogas/waste gas mixture containing only a small quantity of methane is discharged via a waste gas chimney. This is carried out until the methane concentration has dropped to a lower limit at which virtually no methane is contained in the biogas/waste gas mixture any more. Afterwards the fermenter is purged not with waste gas containing carbon dioxide but with fresh air, and discharging the waste gas/biogas/fresh air mixture via the waste gas chimney is continued until the carbon dioxide concentration in the waste gas/biogas/fresh air mixture has dropped to a first limit. Only then the fermenter is switched over for composting. After termination of the composting process, the fermenter may be opened in order to unload the spent biomass and again charge the fermenter with fresh biomass. As a result of composting following fermentation, it is possible to open the fermenter for its unloading and reloading in the absence of any risk.
In accordance with a preferred aspect of the invention, the biogas/waste gas mixture is not emitted to the environment via the waste gas chimney when the upper limit of the methane concentration is reached, but is fed to a waste gas flare and burnt there. Optionally the waste gas flare may be supplied with additional fuel, so that combustion will take place in any case. Combustion of the biogas/waste gas mixture is performed until the methane concentration in the biogas/waste gas mixture becomes less than a medium limit that is situated between the upper and lower limits.
In accordance with preferred aspects of the invention, the composting process is controlled by adjusting the quantity and/or the temperature of the fresh air supplied via the fresh air line, to thus obtain an optimal process medium.
In accordance with a preferred aspect of the invention, the gas mixtures discharged from the fermenter are filtered. As a result of filtering, substances possibly detrimental to the consumers, which might result in clogging of valves, for instance, are removed to the largest possible extent.
In accordance with a preferred aspect of the invention, an explosive biogas/air mixture is safely prevented from being formed during start-up.
This fermenter which has been started again is connected to the biogas line at a fourth methane concentration limit, which is equal to the upper limit.
The waste gas for purging the fermenter is provided, for example, by an internal combustion engine.
In accordance with a preferred embodiment of the invention, the waste gas containing carbon dioxide is provided from a biogas processing means disposed downstream of the at least one fermenter.
Advantageous aspects of the invention are disclosed herein.
Further details, features and advantages of the invention will become evident from the following description of exemplary embodiments with reference to the drawings, in which: